Majority of conventional lenses are spherical because spherical lenses are easy to manufacture and production cost is low. However, the spherical lenses cannot make parallel rays converge into one point due to aberration. The aberration caused by spherical lenses increases as the aperture size of the spherical lenses increases. Thus, multiple lenses have to be combined to correct the problems. Since many lenses are assembled, there are limitations in achieving high performance especially for a large-aperture lens system. The large aperture lens system using spherical lens requires multiple additional large aperture lenses to correct aberration, which make the lens system bulky and impossible to use in compact portable systems. Alternatively, a single lens can be fabricated to have an aspherical surface in order to make incident light converge into one point or satisfy selected focusing property requirements. The aspherical lens can reduce volume and weight of an optical system and provide high optical performance. For this reason, aspherical lenses have been demanded for beam projector, projection TV, CD-ROM pickup, DVD player, laser printer, laser scanning unit, etc. However, conventional aspherical lenses involve with a complicated time-consuming fabrication process with high cost and have difficulty in achieving actually required lens specification.
Fresnel type lenses are a good solution for making large lenses without handling large and heavy materials. However, the quality of the conventional Fresnel lenses is not as good as the conventional spherical lens. The conventional Fresnel lenses offer only procedure reducing thickness.
Gradient index lenses can be another choice for reducing the volume of a lens system. The gradient index lens uses a flat surface material with a gradually varying refractive index and functions as a conventional refractive lens. Using the gradient index of material and geometrical variation together, the aberration of the lens system can be reduced. Although the gradient index lens gives significant reduction in the aberration and volume, it is still expensive and difficult to fabricate.
Many optical systems require a lens or a lens system having variable optical properties including a variable focal length and/or a variable optical axis. A most widely used conventional variable focal length lens system is the one using two refractive lenses. It has complex driving mechanisms to control the relative positions of refractive lenses and a slow response time. Regarding the variable optical axis, most conventional optical systems use attitude control systems with a macroscopic servo mechanism in order to provide the variable optical axis. These attitude control systems tend to be slow, bulky, and expensive to operate.
Other lenses have been made of electrically variable refractive index media to create either a conventional lens or a gradient index lens by means of a voltage gradient. The electrically variable refractive index allows the focal length of the lenses to be voltage controlled. Among them, the most advanced variable focal length lens is a liquid crystal variable focal length lens, which has a complex mechanism to control the focal length. Its focal length is changed by modulating the refractive index. Unfortunately, it has a slow response time typically on the order of hundreds of milliseconds. Even though the fastest response liquid crystal lens has the response time of tens of milliseconds, it has small focal length variation and low focusing efficiency.
MEMS devices using micromirrors can provide many demanding features of lens systems such as reduced volume, easy and mass production, and fast response time. In the prior art of micromirror array like Digital Micromirror Device (DMD), the micromirrors are arranged in the flat surface and actuated by digital control of a voltage. In addition to satisfying the demanding feature of lens systems, the DMD has large rotations of the micromirrors, a low driving voltage, and compatibility with known semiconductor electronics technologies. However, it has only single DOF (Degree Of Freedom); rotation about an axis with two level positions. So, the DMD is merely the array of optical switches rather than a lens.
The Micromirror Array Lens of the present invention works as various types of lenses with variable optical properties in addition to the advantages of the DMD devices by using a plurality of the micromirror motions.